Capacitive deionization devices and systems have historically employed high surface area carbon materials such as carbon aerogels (see Farmer J et al., J Electrochem Soc 143:159 (1996) and Gabelich C et al., Environ Sci Technol 36:3010 (2002)) or other porous carbon electrodes. (See Johnson A M et al., J Electrochem Soc 118:510 (1971)). Such systems are effective at treating already-softened water (i.e., where only Na+ and other single valent cations are present). However, such systems generally fail to effectively remove Ca2+ and other multivalent cations from hard water.
Many metal oxides have traditionally been used in the manufacture of electrochemical electrodes including but not limited to, RuO2, MnO2, V2O5 and NiO. (Jang J et al., J Electrochem Soc 153:A321 (2006); Pang S et al., Electrochem Soc 147:444 (2000); and Liu et al., J Electrochem Soc 143:124 (1996)). Also, many of these metal oxide electrochemical electrodes undergo faradaic reactions as part of their capacitive behavior (i.e., psuedocapacitors). Such metal oxides also undergo redox reactions yielding mixed oxidation states, which may be undesirable in given systems of use. Metal oxides may be more costly t insulating oxides.
However, there exists a long felt need for unproved electrodes, water softeners, water treatment, desalination, and deionization equipment to effectively remove single and particularly multivalent cations (and anions) from hard water.